1. Field of the Invention
The present invention relates to a radio network controller used in a mobile communication system in which same downlink signals can be transmitted to a mobile station via a plurality of base stations, and a mobile communication control method for controlling the mobile communication system.
2. Description of the Related Art
In a conventional second-generation mobile communication system, such as PDC (Personal Digital Cellular) system, a “hard handover method” is employed when a mobile station moves between cells (sectors) during mobile communications. In this hard handover method, a radio communication link between the mobile station and a base station which manages the origination cell (sector) of the handover is firstly released, and then a radio communication link between the mobile station and a base station which manages the destination cell (sector) of the handover is set up.
On the other hand, in a CDMA (Code Division Multiple Access) mobile communication system, a “diversity handover method” is employed when a mobile station moves between cells (sectors) during mobile communications. In this diversity handover method, a radio communication link between the mobile station and a base station which manages the destination cell (sector) of the handover is set up, before a radio communication link between the mobile station and a base station which manages the origination cell (sector) of the handover is released.
In the diversity handover method, when the mobile station is located in a predetermined area near the border between cells (sectors), the mobile station is configured to set up a plurality of radio communication links so as to communicate with a plurality of base stations which manage those cells (sectors).
Further, in the diversity handover method, the mobile station is configured to perform “selective combining process” during the diversity handover process. In the selective combining process, the mobile station selects and receives the most reliable information from among information included in a plurality of downlink signals transmitted from a plurality of base stations, based on reliability information corresponding to reception qualities of those downlink signals (e.g. CRC check results, reception power, reception SIR, or the like).
However, in such conventional mobile communication systems, even if same downlink signals are transmitted via a plurality of base stations, the mobile station may select information included in a particular downlink signal according to the reception qualities of the downlink signals in the mobile station or location information of the mobile station. As a result, there has been a problem that transmitting those downlink signals, which are not selected by the mobile station, is wasted.
Further, in such conventional mobile communication systems, except during the diversity handover process, downlink signals are not transmitted via the second base station managing the second cell adjacent to the first cell in which the mobile station is located. Therefore, there has been a problem that the mobile station cannot perform selective combining process even when the reception quality of the downlink signal in the first cell is damaged or the mobile station is located away from the first base station, such as an area near the border between the first cell and the second cell.